Method for producing magnesium alloy plate and magnesium alloy plate

ABSTRACT

The present invention provides a method for producing a magnesium alloy sheet capable of producing a magnesium alloy sheet having excellent plastic workability such as press workability. The method of the present invention includes rolling a magnesium alloy blank with a reduction roll. The rolling includes controlled rolling performed under the following conditions (1) and (2) wherein M (% by mass) is the Al content in a magnesium alloy constituting the blank:
         (1) the surface temperature Tb (° C.) of the magnesium alloy blank immediately before insertion into the reduction roll satisfies the following expression:       

       8.33× M +135≦ Tb ≦8.33× M +165
 
     wherein 1.0≦M≦10.0; and
         (2) the surface temperature Tr of the reduction roll is 150° C. to 180° C.

TECHNICAL FIELD

The present invention relates to a method for producing a magnesiumalloy sheet and a magnesium alloy sheet produced by the method. Inparticular, the present invention relates to a method for producing amagnesium alloy sheet capable of producing a magnesium alloy sheet withexcellent press workability.

BACKGROUND ART

Magnesium alloys are low-density metals and have high strength and highrigidity and are thus attract attention as lightweight structuralmaterials. In particular, expanded materials are excellent in mechanicalproperties such as strength and toughness, and thus expected to bepopularized in future. The properties of magnesium alloys are changed bychanging the types and amounts of the metal elements added. Inparticular, alloys (for example, AZ91 on the basis of the ASTMstandards) having high aluminum contents have high corrosion resistanceand high strength and are in great demand as expanded materials.However, magnesium alloys have low plastic workability at roomtemperature because of the hexagonal close-packed crystal structurethereof, and thus press working of sheet materials are carried out at ahigh sheet temperature of 200° C. to 300° C. Therefore, the developmentof magnesium alloy sheets capable of stable working at as low atemperature as possible has been desired.

In producing a magnesium alloy sheet, various methods can be used.However, for example, die casting and thixomolding have difficulty inproducing a thin alloy sheet and have the problem of producing manycrystals in a magnesium alloy sheet produced by rolling an extrudedmaterial of a billet, increasing the crystal grain size, or rougheningthe surface of the sheet. In particular, in a magnesium alloy with ahigh Al content, crystals or segregation easily occurs in casting, andthere is thus the problem of leaving crystals or segregated substancesin the final ally sheet even after a heat treatment step and a rollingstep after casing, thereby causing a starting point of breakage duringpress working.

In a typical example of conventional known methods for producing amagnesium alloy sheet, a magnesium alloy blank is pre-heated to 300° C.or more and then rolled with a reduction roll at room temperature, thepre-heating and rolling being repeated.

Also, as a technique for producing a magnesium alloy sheet containingfine crystal grains for improving plastic workability, the methoddisclosed in Patent Document 1 is known. This method includes rolling amagnesium alloy blank at a surface temperature of 250° C. to 350° C.with a reduction roll at a surface temperature of 80° C. to 230° C.

Other known techniques for improving the plastic workability ofmagnesium alloy sheets are disclosed in Patent Documents 2 to 5.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2005-2378-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2003-27173-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2005-29871-   Patent Document 4: Japanese Unexamined Patent Application    Publication No. 2001-294966-   Patent Document 5: Japanese Unexamined Patent Application    Publication No. 2004-346351

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, the method of repeating pre-heating of a blank at 300° C. ormore and rolling with a reduction roll at room temperature coarsens thecrystal grains of a magnesium alloy in pre-heating and thus degrades theplastic workability of the resultant magnesium alloy sheet.

On the other hand, in the method of Patent Document 1, rolling isperformed for a magnesium alloy sheet at a surface temperature of 250°C. to 350° C., and a plurality of rolling passes under this conditionsremoves the working strain produced in the alloy sheet in the lastrolling pass. Therefore, working strain is not accumulated in the sheetwith a final thickness, and the crystal grains of the magnesium alloysheet are not sufficiently made fine in some cases. As a result, theplastic workability of the resultant magnesium alloy sheet cannot besufficiently improved.

Patent Document 2 discloses a method for producing a magnesium alloythin sheet containing AZ91. However, the document does not specify aspecific characteristic value of mechanical strength and pressformability of the magnesium alloy thin sheet.

Patent Document 3 discloses an AZ91 alloy sheet material. PatentDocument 3 also discloses an example of a tensile test in whichsuperplasticity was expressed under conditions including 300° C. and astrain rate of 0.01 (s⁻¹), and an elongation of 200% was recorded.However, the document does not specify plastic workability and tensileproperties at the temperature (250° C. or less) of actual press formingof the sheet material, and also does not describe an example of pressforming.

Patent Documents 4 and 5 also do not disclose specific values of tensileproperties.

Furthermore, the above-descried cited references 1 to 5 do not disclosethat the amounts of crystals and segregation produced in a magnesiumalloy during casing are decreased to improve plastic workability,particularly press workability.

Accordingly, an object of the present invention is to provide a methodfor producing a magnesium alloy sheet capable of producing a magnesiumalloy sheet having excellent plastic workability such as pressworkability.

Another object of the present invention is to provide a magnesium alloysheet having excellent plastic workability such as press workability.

A further object of the present invention is to provide a magnesiumalloy sheet having high strength and elongation and excellent pressworkability using a twin-roll cast raw material.

Means for Solving the Problems

A method for producing a magnesium alloy sheet of the present inventionincludes rolling a magnesium alloy blank with a reduction roll. Therolling includes controlled rolling performed under the followingconditions (1) and (2) wherein M (% by mass) is the Al content in amagnesium alloy constituting the blank.

(1) The surface temperature Tb (° C.) of the magnesium alloy blankimmediately before insertion into the reduction roll satisfies thefollowing equation:

8.33×M+135Tb8.33×M+165

wherein 1.0≦M≦10.0.

(2) The surface temperature Tr of the reduction roll is 150° C. to 180°C.

When the reduction roll temperature Tr and the surface temperature Tb ofthe blank were specified as described above, rolling can be performedwithin a range causing no recrystalliztaion of the crystal gains of themagnesium alloy. Consequently, coarsening of the crystal grains of thealloy can be suppressed, and rolling can be performed while preventingthe occurrence of cracks in the surface of the blank.

A magnesium alloy sheet of the present invention is produced by themethod for producing the magnesium alloy sheet of the present invention.

The magnesium alloy sheet produced by the method of the presentinvention has high plastic workability and is capable of effectivelydecreasing the occurrence of cracks during working.

The present invention will be described in further detail below.

(Gist of Method of the Invention)

The method of the present invention is used for rolling a magnesiumblank to produce a magnesium alloy sheet having a predeterminedthickness. In this method, typically, the blank after casting is roughlyrolled under conditions other than the conditions of controlled rollingand then finish-rolled under the above-described controlled conditions.In other words, the method of the present invention is applied to notonly controlled rolling performed over the entire region of the rollingstep after casing but also controlled rolling performed in a portion ofthe region.

(Surface Temperature Tr of Reduction Roll)

The surface temperature Tr of the reduction roll is 150° C. to 180° C.At the surface temperature lower than 150° C., when the rollingreduction per pass is increased, fine crocodiling may occur in adirection perpendicular to the transfer direction of the blank duringrolling of the blank. On the other hand, at the temperature higher than180° C., strain of the blank, which has been accumulated in previousrolling, is removed by recrystallization of the alloy crystal grains,thereby decreasing the amount of working strain and causing difficultyin making fine the crystal grains.

The surface temperature of the reduction roll can be controlled by amethod of disposing a heating element such as a heater in the reductionroll or a method of spraying hot air onto the surface of the reductionroll.

(Surface Temperature Tb of Blank)

The surface temperature Tb (° C.) of the magnesium alloy blankimmediately before insertion into the reduction roll satisfies thefollowing equation:

8.33×M+135≦Tb≦8.33×M+165

wherein 1.05≦M≦10.0.

In other words, the lower limit of the surface temperature Tb is about140° C., and the upper limit is about 248° C. The temperature Tb dependson the Al content N (% by mass) in the magnesium alloy. Specifically,for ASTM standard AZ31, the temperature Tb may be set to about 160° C.to 190° C., while for AZ91, the temperature Tb may be set to about 210°C. to 247° C. At the temperature lower than the lower limit of eachcomposition, like in a reduction roll at a lower surface temperature,fine crocodiling may occur in the direction perpendicular to thetransfer direction of the blank. While at the temperature higher thanthe upper limit of each composition, strain of the blank, which has beenaccumulated in previous rolling, is removed by recrystallization of thealloy crystal grains during the rolling work, thereby decreasing theamount of working strain and causing difficulty in making fine thecrystal grains.

Even when the surface temperature Tb of the blank falls in theabove-described specified range, for example, with the reduction rollsurface at room temperature, the surface temperature of the blank isdecreased at the time of contact with the roll, thereby producing cracksin the surface of the blank. By specifying not only the surfacetemperature of the reduction roll but also the surface temperature ofthe blank, the occurrence of cracks can be effectively suppressed.

(Rolling Reduction of Controlled Rolling)

The total rolling reduction of controlled rolling is preferably 10% to75%. The total rolling reduction is represented by (thickness of sheetbefore controlled rolling−thickness of sheet after controlledrolling)/(thickness before controlled rolling)×100. When the totalrolling reduction is less than 10%, the working strain of a workingobject is decreased, and the effect of making fine the crystal grains isdecreased. Conversely, when the total rolling reduction exceeds 75%, theworking strain near the surface of the working object is increased, andthus cracking may occur. For example, when the final thickness of thesheet is 0.5 mm, a sheet material of 0.56 to 2.0 mm in thickness may besubjected to controlled rolling. More preferably, the total rollingreduction of controlled rolling ranges from 20% to 50%.

Furthermore, the rolling reduction per pass (average rolling reductionper pass) of controlled rolling is preferably about 5% to 20%. When therolling reduction per pass is excessively low, efficient rolling isdifficult, while when the rolling reduction per pass is excessivelyhigh, defects such as cracks easily occur in the rolling object.

(Other Rolling Conditions)

A plurality of the above-mentioned controlled rolling passes isperformed. Among the plurality of passes, at least one pass ispreferably performed in a direction reverse to the rolling direction ofthe other passes. By rolling in the reverse direction, working strain iseasily uniformly introduced into the working object in comparison to aplurality of rolling passes in the same direction. As a result,generally, variations in the crystal grain size after final heattreatment performed after the controlled rolling can be decreased.

In addition, as described above, rolling of the blank generally includesrough rolling and finish rolling. In this case, at least the finishrolling is preferably the controlled rolling. In view of furtherimprovement in plastic workability, the controlled rolling is preferablyperformed over the entire region of the rolling step. However, thefinish rolling is preferably the controlled rolling because the finishrolling is most concerned in suppressing coarsening of the crystalgrains of the final resulting magnesium alloy sheet.

In other words, rough rolling other than finish rolling is restricted bythe rolling conditions of controlled rolling. In particular, the surfacetemperature of the blank to be roughly rolled is not particularlylimited. The surface temperature and rolling reduction of the blank tobe roughly rolled may be controlled to select conditions for decreasingas much as possible the crystal grain size of the alloy sheet. Forexample, when the thickness of the blank before rolling and thethickness of the final sheet are 4.0 mm and 0.5 mm, respectively, theblank may be roughly rolled to a thickness of 0.56 mm to 2.0 mm and thenfinish-rolled.

In particular, under the rough rolling conditions in which the surfacetemperature of the reduction roll is set to 180° C. or more, and therolling reduction per pass is increased, it is expected that the workingefficiency of rough rolling is increased. In this case, for example, therolling reduction per pass is preferably 20% to 40%. However, even whenthe surface temperature of the reduction roll is 180° C. or more, thesurface temperature is preferably 250° C. or less in order to suppressrecrystallization of the alloy crystal grains.

In addition, in the rough rolling step, preferably, the surfacetemperature Tb of the blank immediately before the insertion into thereduction roll is 300° C. or more, and the surface temperature Tr of thereduction roll is 180° C. or more. In this case, the sheet after roughrolling has an improved surface state without edge cracks. When theblank surface temperature and the roll surface temperature are 300° C.or less and less than 180° C., respectively, the rolling reductioncannot be increased, thereby decreasing the working efficiency of therough rolling step. Although the upper limit of the blank surfacetemperature is not particularly limited, the surface state of the sheetafter rough rolling may be degraded at a higher surface temperature.Therefore, the surface temperature is preferably 400° C. or less.Although the upper limit of the surface temperature of the roll forrough rolling is not particularly limited, the roll itself may bedamaged by thermal fatigue at a higher temperature. Therefore, thesurface temperature of the roll is preferably 300° C. or less.

When the rolling reduction per pass of rough rolling within theabove-described temperature range is 20% to 40%, variation in grain sizeof the magnesium alloy sheet finish-rolled after rough rolling can bedesirably decreased. When the rolling reduction per pass of roughrolling is less than 20%, the effect of decreasing variation in grainsize after rolling is decreased, while when the rolling reductionexceeds 40%, edge cracks occur at the edge of the magnesium alloy sheetduring rolling. The number of passes (pass number) of rolling with arolling reduction within in this range is preferably at least 2 becauseone pass of rolling exhibits the low effect.

Furthermore, in rolling (initial rough rolling) of the cast blank, it ispreferred to increase the temperature of the blank and increase therolling reduction within the above-described rolling reduction range sothat in rough rolling immediately before finish rolling, the blanktemperature is about 300° C., and the rolling reduction is about 20%.

Rough rolling under the above-mentioned conditions can improve theplastic workability of the magnesium alloy sheet obtained by finishrolling in succession to the rough rolling. Specifically, it is possibleto improve the surface state of the alloy sheet, suppress the occurrenceof edge cracks, and decrease variation in crystal grain size of thealloy sheet. Also, the amount of segregation in the magnesium alloysheet can be decreased.

(Blank)

The blank used in rolling in the present invention may be composed of amagnesium alloy containing Al, and the other components are notparticularly limited. For example, a variety of materials, such as ASTMstandard AZ, AM, and AS alloys, can be preferably used.

A method for producing the magnesium alloy blank is not particularlylimited. For example, a blank prepared by an ingot casting method, anextrusion method, or a twin-roll casting method, may be used.

In the ingot casting method for producing the blank, for example, aningot of about 150 mm to 300 mm in thickness is cast, and the cast ingotis hot-rolled after cutting of the surface of the cast ingot. The ingotcasting method is suitable for mass production and capable of producingthe blank at low cost.

In the extrusion method for producing the blank, for example, a billetof about 300 mm in diameter is cast, and the resultant billet isre-heated and then extruded. The extrusion method includes strongcompression of the billet during extrusion and thus can crush crystalsin the billet to some extent, the crystals easily causing startingpoints of cracking during subsequent rolling of the blank and plasticworking of the rolled material.

In twin roll casting method for producing the blank, a melt is suppliedfrom an inlet between a pair of rolls with the peripheral surfacesopposed to each other, and a solidified blank is delivered as a thinsheet from an outlet.

Among the blanks prepared by these three methods, the blank prepared bythe twin roll casting method is preferably used. The twin-roll castingmethod is capable of quick solidification using twin rolls and thuscauses little internal defects such as oxides and segregation in theresultant blank. In particular, after a rolled sheet having a finalthickness of 1.2 mm or less is produced, defects which adversely affectsubsequent plastic working such as press working can be eliminated. Morespecifically, crystals of 10 μm or more in diameter do not remain in therolled sheet. In addition, a blank containing a small amount of crystalscan be obtained regardless of the alloy composition such as AZ31 orAZ91. Furthermore, a thin sheet can be obtained using a materialdifficult to work, and thus the number of subsequent rolling steps ofthe blank can be decreased to decrease the cost.

(Other Working Conditions)

As another working condition, if required, solution treatment of theblank may be performed before rolling. The conditions of the solutiontreatment include, for example, 380° C. to 420° C. and about 60 minutesto 600 minutes and preferably 390° C. to 410° C. and about 360 minutesto 600 minutes. This solution treatment can decrease segregation. Inparticular, a magnesium alloy having a high Al content corresponding toAZ91 is preferably subjected to solution treatment for a long time.

If required, strain relief annealing may be performed in the rollingstep (which may not be controlled rolling). The strain relief annealingis preferably performed between passes in a portion of the rolling step.The stage in the rolling step in which the strain relief treatment isperformed and the number of strain relief treatments may beappropriately selected in view of the amount of strain accumulated inthe magnesium alloy sheet. The strain relief treatment permits smoothrolling in the subsequent pass. The strain relief treatment conditionsinclude, for example, 250° C. to 350° C. and about 20 minutes to 60minutes.

Furthermore, the rolled material after the whole rolling work ispreferably finally annealed. Since the crystal structure of themagnesium alloy sheet after finish rolling contains sufficientlyaccumulated working strain, fine recrystallization occurs in finalannealing. Namely, even the alloy sheet which has been finally annealedto relieve strain has a fine recrystallized structure and is thusmaintained in a high-strength state. Also, when the structure of thealloy sheet is previously recrystallized, a large change in the crystalstructure, such as coarsening of the crystal grains in the structure ofthe alloy sheet, does not occur after plastic working at a temperatureof about 250° C. Therefore, in the finally annealed magnesium alloy, aportion plastically deformed by plastic working can be improved instrength by work hardening, and a portion not plastically deformed canbe maintained at the strength before the working. The final annealingconditions include 200° C. to 350° C. and about 10 minutes to 60minutes. Specifically, when the Al content and zinc content in amagnesium alloy are 2.5 to 3.5% and 0.5 to 1.5%, respectively, the finalannealing is preferably performed at 220° C. to 260° C. for 10 minutesto 30 minutes. When the Al content and zinc content in a magnesium alloyare 8.5 to 10.0% and 0.5 to 1.5%, respectively, the final annealing ispreferably performed at 300° C. to 340° C. for 10 minutes to 30 minutes.

(Centerline Segregation)

In the Sheet Produced from a Twin-Roll Cast Material, Segregation Occursin a central portion in the thickness direction during casting. In anAl-containing magnesium alloy, a segregated substance is anintermetallic compound mainly composed of the composition Mg₁₇A₁₂, andthe higher the impurity content in the magnesium alloy, the moresegregation occurs. For example, in an ASTM standard AZ alloy, theamount of segregation in AZ91 having an Al content of about 9% by massis larger than that of AZ31 having an Al content of about 3% by mass.Even in the AZ91 causing larger segregation, the length of segregationin the thickness direction of the magnesium alloy sheet can be dispersedto 20 μm or less by solution treatment under appropriate conditionsbefore the above-described rough rolling step and finish rolling. Theexpression “segregation is dispersed” means that linear segregation isdivided in the thickness direction and in the length direction.

The criterion for the length of segregation in the thickness directionwhich causes no trouble in press working is 20 μm or less. Therefore,the length of segregation in the thickness direction is preferablyfurther decreased to be smaller than 20 μm, and it is thus supposed thatthe strength property is improved by dispersing the maximum length ofsegregation to a length smaller than the crystal grain size of the basemetal.

(Mechanical Properties of Magnesium Alloy Sheet)

When strain is accumulated in the rolling step and not removed by a heattreatment in producing the magnesium alloy sheet, tensile strength canbe easily controlled to 360 MPa. However, in this case, it is difficultto control the elongation of the alloy sheet to 10% or more.Specifically, when the elongation at breakage at room temperature isless than 15%, plastic workability is low, and damages such as cracks orflaws occur in press forming at a temperature of as low as 250° C. orless. On the other hand, when the elongation at breakage of themagnesium alloy sheet at room temperature is 15% or more, the elongationat breakage at 250° C. of the alloy sheet is 100% or more, andsubstantially no damage such as surface cracks or flaws occurs in themagnesium alloy sheet in press forming. The method for producing themagnesium alloy sheet of the present invention is effective in producinga magnesium alloy sheet having the above-described mechanicalproperties. In particular, even by using a magnesium alloy having a highAl content M of 8.5 to 10.0% by mass (further having a zinc content of0.5 to 1.5% by mass), a magnesium alloy sheet having a tensile strengthof 360 MPa or more, a yield strength of 270 MPa or more, and anelongation at breakage of 15% or more at room temperature can beproduced. The method for producing the magnesium alloy sheet of thepresent invention can produce a magnesium alloy sheet having a yieldratio of 75% or more.

The magnesium alloy sheet is preferably plastically worked in atemperature range in which the mechanical properties of the alloy sheetare not significantly changed by recrystallization in the structure ofthe alloy sheet during the plastic working. For example, a magnesiumalloy sheet containing 1.0 to 10.0% by mass of Al is preferablyplastically worked at a temperature of about 250° C. or less. In themethod for producing the magnesium alloy sheet of the present invention,a magnesium alloy sheet having an Al content M of 8.5 to 10.0% by massand a zinc content of 0.5 to 1.5% by mass can be made to have a tensilestrength of 120 MPa or more and an elongation at breakage of 80% or moreat 200° C. and a tensile strength of 90 MPa or more and an elongation atbreakage of 100% or more at 250° C. Therefore, the method is suitablefor plastic working, particularly high deformation such as pressforming. Furthermore, in the method for producing the magnesium alloysheet of the present invention, a magnesium alloy sheet corresponding toAZ31 can be made to have a tensile strength of 60 MPa or more and anelongation at breakage of 120% or more at 250° C.

ADVANTAGE OF THE INVENTION

As described above, the method of the present invention exhibits thefollowing advantages:

In the method of the present invention, the temperature of the blank andthe temperature of the reduction roll in rolling are specified so thatrolling can be performed within a range causing no recrystallization ofthe crystal grains of the magnesium alloy used. It is thus possible tosuppress coarsening of the crystal gains of the alloy and permittingrolling causing little cracking in the surface of the blank used. Also,it is possible to decrease the amount of segregation in a centralportion of the blank and decrease variation in grain size of the crystalgrains.

In particular, when the blank prepared by the twin-roll casting methodis rolled, crystals serving as starting points of cracking little occur,thereby producing no crack or permitting plastic working causingsubstantially no cracking.

The magnesium alloy sheet of the present invention has the followingcharacteristics:

The magnesium alloy sheet of the present invention has very excellentplastic workability because it is composed of fine crystal grains.

The magnesium alloy sheet of the present invention simultaneouslysatisfies a tensile strength of 360 MPa or more, a yield strength of 270MPa or more, and an elongation at breakage of 15% or more and thusproduces no problem even in press forming.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below.

Test Example 1

A magnesium alloy blank having a thickness of 4 mm and a compositioncorresponding to AZ31 containing Mg, 3.0% of Al, and 1.0% of Zn (% bymass) was prepared by the twin-roll continuous casting method. The blankwas roughly rolled to a thickness of 1 mm to prepare a roughly rolledsheet having an average crystal grain size of 6.5 μm. Rough rolling wasperformed by pre-heating the blank to 250° C. to 350° C. and thenrolling the blank with a reduction roll at room temperature. The averagecrystal grain size was determined by the calculation expressiondescribed in JIS G0551. Next, the roughly rolled sheet was finish-rolledto a thickness of 0.5 mm under various conditions. Each of thefinish-rolled sheets was finally heat-treated at 250° C. for 30 minutes,and a disk having a diameter of 92 mm was cut out from each heat-treatedmaterial and used as an evaluation sample.

Next, the observation surface of each sample was buffed (diamondabrasive grains #200) and then etched to observe the structure andmeasure the average crystal grain size in the field of view of anoptical microscope with a magnification of 400×.

Furthermore, each sample was drawn using a cylindrical punch and a diehaving a cylindrical hole engaging with the punch under the followingconditions:

Mold set temperature: 200° C.

Punch diameter: 40.0 mm (tip R: Rp=4 mm)

Die hole diameter: 42.5 mm (shoulder R: Rd=4 mm)

Clearance: 1.25 mm

Molding rate: 2.0 mm/min

Drawing ratio: 2.3

Here, Rp is the radius of a curve constituting the punch outer peripheryin a longitudinal section of the punch tip and Rd is the radius of acurve constituting the die hole opening in a longitudinal section of thedie. The drawing ratio is defined as (diameter of sample/diameter ofpunch).

The finish rolling conditions and the test results are summarized inTable I. In this table, each designation means the following:

Sheet temperature: the surface temperature of the blank immediatelybefore finish rolling.

Roll temperature: the surface temperature of the reduction roll forfinish rolling.

Rolling direction: “Constant” means that all rolling passes wereperformed in the same direction, and “R” means that the rollingdirection was reversed in every rolling pass.

Average rolling reduction per pass: total rolling reduction (50%)/numberof times of rolling from a thickness of 1 mm to a thickness of 0.5 mm.

Sheet surface state: Symbol “A” means no occurrence of cracks orwrinkles in a rolled material; symbol “B”, the occurrence of littlecrocodiling; and symbol “C”, the occurrence of cracks.

Edge crack: Symbol “A” means no occurrence of cracks at the edge of arolled material; symbol “B”, the occurrence of only little cracks; andsymbol “C”, the occurrence of cracks.

Drawability: Symbol “A” means no occurrence of cracks at the corners ofa produced good; Symbol “B”, the occurrence of wrinkles but no crack;and symbol “C”, the occurrence of cracks or breakage.

TABLE I Average Sheet Roll rolling Sheet Average Sample temperaturetemperature Rolling reduction surface Edge crystal grain No. (° C.) (°C.) direction per pass (%) state crack size (μm) Drawability 1-1 140 175R 8 C C 4.1 C 1-2 150 173 R 7 B B 4.1 B 1-3 160 168 R 8 A A 4.2 A 1-4170 170 R 6 A A 4.3 A 1-5 180 169 R 7 A A 4.3 A 1-6 190 175 R 8 A A 4.5A 1-7 200 178 R 7 A A 5.6 C 1-8 210 176 R 7 A A 6.0 C 1-9 220 175 R 7 AA 7.7 C  1-10 230 175 R 8 A A 8.0 C  1-11 175 166 R 14 A B 3.8 A  1-12180 168 R 14 A B 3.7 A  1-13 176 171 R 22 B B 3.4 B  1-14 178 174 R 20 AB 3.5 B  1-15 170 168 Constant 7 A B 4.4 B  1-16 180 171 Constant 7 A B4.5 B Rolling direction: “R” means the reverse rolling direction.

This table indicates that all samples finish-rolled under the controlledrolling conditions specified in the present invention have small averagegrain sizes, neither edge crack nor fine crack in the surfaces, andexcellent drawability. The crystals in the samples according to thepresent invention have a size of 5 μm or less.

Test Example 2

Next, the same blank having a thickness of 4 mm as in Test example 1 wasprepared and then roughly rolled to predetermined thicknesses to prepareroughly rolled sheets having different thicknesses. The rough rollingwas performed by pre-heating the blank at 250° C. to 350° C. and thenrolling the blank with a reduction roll at room temperature. Each of theroughly rolled sheets was finish-rolled to a final sheet thickness of0.5 mm with different total rolling reductions to prepare finish-rolledsheets. The finish rolling was performed under the conditions in whichthe surface temperature of each roughly rolled sheet was 160° C. to 190°C. immediately before finish rolling, and the surface temperature afinish reduction roll was controlled in the range of 150° C. to 180° C.Next, each of the finish-rolled materials was heat-treated at 250° C.for 30 minutes by the same method as in Test example 1 to form anevaluation sample.

For these samples, the measurement of the average crystal grain size,the evaluation of the sheet surface state, the evaluation of edgecracks, and the overall evaluation of these evaluation results werecarried out by the same methods as in Test example 1. The rollingreduction per pass and the total rolling reduction of finish rolling,and the evaluation results are shown in Table II. In this table, theterms “Sheet surface state” and “Edge crack” mean the same as in Testexample 1. The term “Total rolling reduction” means the total rollingreduction of finish rolling from the thickness of the roughly rolledmaterial to the final sheet thickness, i.e., the total rolling reductionof rolling at a sheet surface temperature of 160° C. to 190°. However,the numerical value in parentheses shown in No. 2-1 indicates that theroughly rolled sheet was finish-rolled at a sheet surface temperature of220° C.

TABLE II Average rolling Sheet Sample reduction per Total rollingreduction surface Edge Average crystal Overall No. pass (%) at 160 to190° C. (%) state crack grain size (μm) evaluation 2-1  7 0 (220° C.) AA 7.7 C 2-2  4 4 A A 6.5 C 2-3  8 8 A A 6.2 C 2-4  5 10 A A 5.0 A 2-5  818 A A 4.8 A 2-6  7 20 A A 4.7 A 2-7  9 24 A A 4.6 A 2-8  12 24 A A 4.5A 2-9  10 28 A A 4.8 A 2-10 14 28 A B 4.6 A 2-11 28 28 B B 4.6 A 2-12 2828 B B 4.5 B 2-13 16 32 B B 4.5 B 2-14 9 35 A A 4.4 A 2-15 8 40 A A 4.4A 2-16 8 45 A A 4.4 A 2-17 15 45 A A 4.0 A 2-18 8 50 A A 4.3 A 2-19 1550 B B 3.9 B 2-20 22 50 B B 3.7 B 2-21 9 60 B A 3.9 B 2-22 12 65 B B 3.8B 2-23 23 70 B B 3.8 B 2-24 15 70 B B 3.7 B 2-25 10 76 C C 3.7 C 2-26 1080 C C 3.6 C

This table indicates that the samples with total rolling reductions of10% to 75% exhibit excellent results in the overall evaluation.

Test Example 3-1

A magnesium alloy blank having a thickness of 4 mm and a compositioncorresponding to AZ91 containing Mg, 9.0% of Al, and 1.0% of Zn (% bymass) was prepared by the twin-roll continuous casting method. The blankwas roughly rolled to a predetermined thickness of 1 mm to prepare aroughly rolled sheet having an average crystal grain size of 6.8 μm. Therough rolling was performed by pre-heating the blank at 300° C. to 380°C. and then rolling the blank with a reduction roll at room temperature.The average crystal grain size was determined by the calculationexpression described in JIS G0551. Next, the roughly rolled sheet wasfinish-rolled to a thickness of 0.5 mm under various conditions. Each ofthe finish-rolled sheets was finally heat-treated at 320° C. for 30minutes, and a disk having a diameter of 92 mm was cut out from eachheat-treated material and used as an evaluation sample.

Next, the observation surface of each sample was buffed (diamondabrasive grains #200) and then etched to observe the structure andmeasure the average crystal grain size in the field of view of anoptical microscope with a magnification of 400×.

Furthermore, each sample was drawn using a cylindrical punch and a diehaving a cylindrical hole engaging with the punch under the sameconditions as in Test Example 1 except that the mold set temperature was250° C. The finish rolling conditions and the test results aresummarized in Table III. In this table, each designation means the sameas in Test Example 1.

TABLE III Sheet Roll Average rolling Sheet Sample temperaturetemperature Rolling reduction per surface Edge Average crystal No. (°C.) (° C.) direction pass (%) state crack grain size (μm) Drawability3-1 190 173 R 7 C C 4.2 C 3-2 200 175 R 8 B B 4.3 B 3-3 210 169 R 8 A A4.3 A 3-4 220 170 R 7 A A 4.3 A 3-5 230 167 R 7 A A 4.4 A 3-6 240 170 R8 A A 4.5 A 3-7 250 178 R 7 A A 5.8 C 3-8 260 175 R 7 A A 6.1 C 3-9 270174 R 7 A A 7.8 C  3-10 280 176 R 8 A A 8.1 C  3-11 225 166 R 15 A B 4.0A  3-12 230 160 R 15 A B 4.1 A  3-13 226 171 R 23 B B 4.1 B  3-14 228174 R 20 A B 3.9 B  3-15 220 169 Constant 8 A B 4.5 B  3-16 230 171Constant 7 A B 4.7 B Rolling direction: “R” means the reverse rollingdirection.

Test Example 3-2

A magnesium alloy blank having a different Al content from that in TestExample 3-1 was used for examining the influences of the blanktemperature and roll temperature in finish rolling by the same method asin Test Example 3-1. The producing conditions other than the finishrolling conditions and the evaluation methods for the magnesium alloysheets were the same as in Test Example 3-1. The Al content of themagnesium alloy blank was 9.8% by mass, and the Zn content thereof was1.0% by mass. The finish rolling conditions and the test results aresummarized in Table N.

TABLE IV Sheet Roll Average rolling Sheet Sample temperature temperatureRolling reduction per surface Edge Average crystal No. (° C.) (° C.)direction pass (%) state crack grain size (μm) Drawability 3-17 190 173R 7 C C 4.3 C 3-18 200 175 R 8 B B 4.3 B 3-19 230 170 R 7 A A 4.4 A 3-20260 175 R 7 A A 6.3 C 3-21 280 176 R 8 A A 8.1 C 3-22 230 175 R 15 A A4.2 A 3-23 230 135 R 15 C B 4.1 C 3-24 230 175 R 25 B B 3.9 B 3-25 230175 Constant 7 A B 4.7 B Rolling direction: “R” means the reverserolling direction.

Tables III and IV indicate that all samples finish-rolled under thecontrolled rolling conditions specified in the present invention exhibitsmall average grains sizes, neither edge crack nor fine crack in thesurfaces, and excellent drawability.

Test Example 4-1

Next, the same blank having a thickness of 4 mm as in Test example 3-1was prepared and then roughly rolled to predetermined thicknesses toprepare roughly rolled sheets having different thicknesses. The roughrolling was performed by pre-heating the blank at 300° C. to 380° C. andthen rolling the blank with a reduction roll at room temperature. Eachof the roughly rolled sheets was finish-rolled to a final sheetthickness of 0.5 mm with different total rolling reductions to preparefinish-rolled sheets. The finish rolling was performed under theconditions in which the surface temperature of each roughly rolled sheetwas 210° C. to 240° C. immediately before finish rolling, and thesurface temperature of a finish reduction roll was controlled in therange of 150° C. to 180° C. Next, each of the finish-rolled materialswas heat-treated at 320° C. for 30 minutes by the method as in TestExample 3-1 to form an evaluation sample.

For these samples, the measurement of the average crystal grain size,the evaluation of the sheet surface state, the evaluation of edgecracks, and the overall evaluation of these evaluation results werecarried out by the same methods as in Test Example 3-1. The rollingreduction per pass and the total rolling reduction of finish rolling,and the evaluation results are shown in Table V. In this table, theterms “Sheet surface state” and “Edge crack” mean the same as in TestExample 1. The term “total rolling reduction” means the total rollingreduction of finish rolling from the thickness of the roughly rolledmaterial to the final sheet thickness, i.e., the total rolling reductionof rolling at a sheet surface temperature of 210° C. to 240°. However,the numerical value in parentheses shown in No. 4-1 indicates that theroughly rolled sheet was finish-rolled at a sheet surface temperature of270° C.

TABLE V Average rolling Sheet Sample reduction per Total rollingreduction surface Edge Average crystal Overall No. pass (%) at 210 to240° C. (%) state crack grain size (μm) evaluation 4-1  7  0(270° C.) AA 7.9 C 4-2  4  4 A A 6.4 C 4-3  8  8 A A 6.3 C 4-4  5 10 A A 5.2 A 4-5 8 18 A A 4.8 A 4-6  7 20 A A 4.8 A 4-7  9 24 A A 4.6 A 4-8  12 24 A A4.5 A 4-9  10 28 A A 4.8 A 4-10 14 28 A B 4.7 A 4-11 28 28 B B 4.7 A4-12 28 28 B B 4.5 B 4-13 16 32 B B 4.5 B 4-14 9 35 A A 4.4 A 4-15 8 40A A 4.4 A 4-16 8 45 A A 4.4 A 4-17 15 45 A A 4.0 A 4-18 8 50 A A 4.5 A4-19 15 50 B B 4.2 B 4-20 20 50 B B 4.1 B 4-21 9 60 B A 4.0 B 4-22 12 65B B 4.0 B 4-23 12 70 B B 3.9 B 4-24 15 70 B B 3.9 B 4-25 8 76 C C 3.9 C4-26 10 80 C C 3.8 C

Test Example 4-2

A magnesium alloy blank having a different Al content from that in TestExample 4-1 was used for examining the influences of the average rollingreduction per pass and total rolling reduction of finish rolling by thesame method as in Test Example 4-1. The producing conditions other thanthe finish rolling conditions and the evaluation method for themagnesium alloy sheets were the same as in Test Example 4-1. The Alcontent of the magnesium alloy blank was 9.8% by mass, and the Zncontent thereof was 1.0% by mass. The finish rolling conditions and thetest results are summarized in Table VI.

TABLE VI Average rolling Sheet Sample reduction per Total rollingreduction surface Edge Average crystal Overall No. pass (%) at 217 to247° C. (%) state crack grain size (μm) evaluation 4-27 8  0(270° C.) AA 8.0 C 4-28 8  8 A A 6.5 C 4-29 8 18 A A 4.8 A 4-30 10 28 A A 4.9 A4-31 28 28 B B 4.6 B 4-32 8 40 A A 4.4 A 4-33 8 50 A A 4.5 A 4-34 22 50B B 4.1 B 4-35 14 65 B B 4.1 B 4-36 10 80 C C 4.0 C

Tables V and W indicate that the samples with total rolling reductionsof 10% to 75% exhibit excellent results in the overall evaluation.

Summary of Test Examples 1 to 4

On the basis of the results of Test Examples 1 to 4, the relationbetween the surface temperature Tb (° C.) of the blank immediatelybefore the insertion into the reduction roll and the Al content M (% bymass) in the magnesium alloy constituting the blank was represented bygraphing. As a result, it was found that when the surface temperature Tbof the blank satisfies the following expression, controlled rolling witha reduction roll at a surface temperature Tr of 150° C. to 180° C.produces a magnesium alloy sheet containing fine crystal grains andhaving excellent plastic workability.

8.33×M+135≦Tb8.33×M+165

wherein 1.0≦M≦10.0.

Test Example 5

Furthermore, magnesium alloy sheets (corresponding AZ31) were producedusing different methods for producing the blank and different rollingconditions. The method for producing the blank and the rollingconditions were as follows:

<Method for Producing Blank>

A1: A blank having a thickness of 4 mm was prepared by twin-rollcontinuous casting.

A2: An ingot having a thickness of about 200 mm was cast, cut at thesurface thereof, and then hot-rolled to prepare a blank having athickness of 4 mm.

<Rolling Method>

B1: In rough rolling (thickness of 4 mm to 1 mm), the blank waspre-heated at 250° C. to 350° C. and then rolled with a reduction rollat room temperature. In controlled rolling as finish rolling (thicknessof 1 mm to 0.5 mm), the surface temperature of the reduction roll was150° C. to 180° C., and the surface temperature of the roughly rolledsheet immediately before the insertion into the reduction roll was 160°C. to 190° C.

B2: The blank was pre-heated at 300° C. to 400° C. and then rolled witha reduction roll at room temperature in all rolling passes (thickness of4 mm to 0.5 mm).

The magnesium alloy sheet was rolled in each of the combinations of theabove-described conditions shown in Table V and then the rolled sheetwas finally heat-treated at 250° C. for 30 minutes. For the resultantmagnesium alloy sheets, the measurement of the average crystal grainsize, the evaluation of the sheet surface state, the evaluation of edgecracks, and the overall evaluation of these evaluation results werecarried out. The results are shown in Table III. The results of theoverall evaluation are shown by symbols “A”, “B”, and “C” in the orderfrom a good level.

TABLE VII Sample Method for Rolling Overall No. producing blank methodevaluation 5-1 A1 B1 A 5-2 A1 B2 C 5-3 A2 B1 B 5-4 A2 B2 C

The results indicate that the predetermined controlled rolling using ablank prepared by twin-roll casting can produce a magnesium alloy sheethaving excellent plastic workability.

Test Example 6

A magnesium alloy blank having a thickness of 4 mm and a compositioncorresponding to AZ31 containing Mg, 3.0% of Al, and 1.0% of Zn (% bymass) was prepared by the twin-roll continuous casting method. The blankwas roughly rolled to a thickness of 1 mm under different conditions toprepare a plurality of roughly rolled sheets. The plurality of roughlyrolled sheets was finish-rolled to a final thickness of 0.5 mm under thesame conditions to prepare magnesium alloy sheets. The finish rollingwas performed under the conditions in which the surface temperature ofeach roughly rolled sheet immediately before finish rolling was 160° C.to 190° C., and the surface temperature of a reduction roll wascontrolled in the range of 150° C. to 180° C. Also, the rollingreduction per pass was controlled to 15%. Each of the finish-rolledmagnesium alloy sheets was heat-treated at 250° C. for 30 minutes andused as an evaluation sample. For each of the samples, the measurementof the average crystal grain size, the evaluation of the sheet surfacestate, and the evaluation of edge cracks were performed by the samemethod as in Test Example 1.

The finish rolling conditions and the test results are summarized inTable VIII. In this table, each designation means the following:

Sheet temperature: the surface temperature of the blank immediatelybefore rough rolling.

Roll temperature: the surface temperature of the reduction roll forrough rolling.

Rolling reduction per pass: rolling reduction of rolling from thicknessof 4 mm to 1.0 m/pass

Sheet surface state: Symbol “A” means no occurrence of cracks orwrinkles in a rolled material; symbol “B”, the occurrence of littlecrocodiling; and symbol “C”, the occurrence of cracks.

The average crystal grain size was determined by the calculationexpression described in JIS G0551.

TABLE VIII Temperature of Temperature of Rolling Sheet Average Sampleroughly rolled rough reduction reduction/ surface Edge crystal grainOverall No. sheet (° C.) roll (° C.) pass (%) state crack size (μm)evaluation 6-1  200 150 10 C B 4.8 C 6-2  200 150 20 C C 4.5 C 6-3  250150 10 B B 4.8 B 6-4  250 180 20 B B 4.6 B 6-5  300 150 10 B A 4.7 B6-6  300 150 20 B B 4.5 B 6-7  300 180 20 A A 4.4 A 6-8  300 200 20 A A4.4 A 6-9  300 250 20 A A 4.3 A 6-10 320 150 20 B A 4.4 B 6-11 320 18020 A A 4.4 A 6-12 320 200 20 A A 4.3 A 6-13 350 150 20 B A 4.4 B 6-14350 200 20 A A 4.5 A 6-15 350 250 20 A A 4.5 A 6-16 380 150 20 B A 4.3 B6-17 380 180 20 A A 4.4 A 6-18 380 250 20 A A 4.5 A 6-19 380 250 30 A A4.3 A 6-20 400 150 20 B A 4.3 B 6-21 400 100 20 B B 4.3 B 6-22 400 50 20B B 4.2 B 6-23 400 25 20 C B 4.2 C 6-24 400 25 30 C C 4.0 C

Test Example 7-1

A magnesium alloy blank having a thickness of 4 mm and a compositioncorresponding to AZ91 containing Mg, 9.0% of Al, and 1.0% of Zn (% bymass) was prepared by the twin-roll continuous casting method. The blankwas roughly rolled to a thickness of 1 mm under different conditions toprepare a plurality of roughly rolled sheets. The plurality of roughlyrolled sheets was finish-rolled to a final thickness of 0.5 mm under thesame conditions to prepare magnesium alloy sheets. The finish rollingwas performed under the conditions in which the surface temperature ofeach roughly rolled sheet immediately before finish rolling was 210° C.to 240° C., and the surface temperature of a reduction roll wascontrolled in the range of 150° C. to 180° C. Also, the rollingreduction per pass was controlled to 15%. Each of the finish-rolledmagnesium alloy sheets was heat-treated at 320° C. for 30 minutes andused as an evaluation sample. For each of the samples, the measurementof the average crystal grain size, the evaluation of the sheet surfacestate, and the evaluation of edge cracks were performed by the samemethod as in Test Example 6. Furthermore, overall evaluation wasconducted on the basis of these evaluation results.

The rough rolling conditions and the test results are summarized inTable IX. In this table, each designation means the same as in TestExample 6.

TABLE IX Temperature of Temperature of Rolling Sheet Average Sampleroughly rolled rough reduction reduction/ surface Edge crystal grainOverall No. sheet (° C.) roll (° C.) pass (%) state crack size (μm)evaluation 7-1  250 150 10 C B 5.6 C 7-2  250 150 20 C C 5.2 C 7-3  280150 10 B B 5.7 B 7-4  280 180 20 B B 5.1 B 7-5  300 150 10 B A 5.8 B7-6  300 150 20 B B 5.0 B 7-7  300 180 20 A A 4.9 A 7-8  300 200 20 A A5.0 A 7-9  300 250 20 A A 4.8 A 7-10 320 150 20 B A 4.9 B 7-11 320 18020 A A 4.8 A 7-12 320 200 20 A A 4.9 A 7-13 350 150 20 B A 4.5 B 7-14350 200 20 A A 4.6 A 7-15 350 250 20 A A 4.7 A 7-16 380 150 20 B A 4.7 B7-17 380 180 20 A A 4.5 A 7-18 380 250 20 A A 4.6 A 7-19 380 250 30 A A4.4 A 7-20 380 300 30 A A 4.4 A 7-21 380 300 35 A A 4.2 A 7-22 400 15020 B A 4.9 B 7-23 400 100 20 B B 4.9 B 7-24 400 50 20 B B 4.7 B 7-25 40025 20 C B 4.5 C 7-26 400 25 25 C C 4.4 C

Test Example 7-2

A magnesium alloy blank having a different Al content from that in TestExample 7-1 was used for examining the influences of the temperature ofthe blank and the roll temperature in rough rolling by the same methodas in Test Example 3-1. The producing conditions other than the roughrolling conditions and the evaluation method for the magnesium alloysheets were the same as in Test Example 7-1. The Al content of themagnesium alloy blank was 9.8% by mass, and the Zn content thereof was1.0% by mass. The finish rolling conditions and the test results aresummarized in Table X.

TABLE X Temperature of Temperature of Rolling Sheet Average Sampleroughly rolled rough reduction reduction/ surface Edge crystal grainOverall No. sheet (° C.) roll (° C.) pass (%) state crack size (μm)evaluation 7-28 250 160 10 C B 5.7 C 7-29 280 180 20 B B 5.2 B 7-30 300160 20 B B 5.0 B 7-31 300 180 20 A A 4.9 A 7-32 300 250 20 A A 4.8 A7-33 320 160 20 B A 4.9 B 7-34 320 200 20 A A 4.9 A 7-35 350 160 20 B A4.5 B 7-36 350 250 20 A A 4.7 A 7-37 380 160 20 B A 4.7 B 7-38 380 30030 A A 4.4 A 7-39 380 320 30 B A 4.1 B 7-40 400 160 20 B A 5.0 B 7-41400 100 20 B B 5.1 B 7-42 400 25 20 C C 4.5 C

Test Example 8

The same AZ31 blank (thickness, 4 mm) as that used in Test Example 6 wasprepared and then roughly rolled to a thickness of 1 mm under differentconditions to prepare a plurality of roughly rolled sheets. The roughlyrolled sheets were finish-rolled to a final sheet thickness of 0.5 mmunder the same conditions to prepare magnesium alloy sheets.

The rough rolling was performed under the conditions in which thesurface temperature of each roughly rolled sheet immediately beforerough rolling was 350° C., and the surface temperature of the roughreduction roll was controlled in the range of 200° C. to 230° C. Duringthe rough rolling, the rolling reduction per pass was changed. On theother hand, the finish rolling was performed under the conditions inwhich the surface temperature of each roughly rolled sheet immediatelybefore finish rolling was 160° C. to 190° C., the surface temperature ofa finish reduction roll was controlled in the range of 150° C. to 180°C., and the rolling reduction per pass in the finish rolling wascontrolled to 15%.

Next, each of the finish-rolled sheets was heat-treated at 250° C. for30 minutes by the same method as in Test Example 1 to form an evaluationsample. For these samples, the measurement of the average crystal grainsize, the evaluation of the sheet surface state, the evaluation of edgecracks, and the evaluation of variation in grain size were performed bythe same methods as in Test Example 6. Furthermore, the overallevaluation based on these evaluation results was carried out. The numberof times of rough rolling with a rolling reduction per pass of 20% to40% and the evaluation results are shown in Table XI. In this table, theterms “Sheet surface state” and “Edge crack” mean the same as in TestExample 6. The term “Number of times of rough rolling with rollingreduction or 20% of 40%” means the number of times of rough rolling witha rolling reduction of 20% to 40% at each time, and the term “Maximumrolling reduction per pass” means the maximum rolling reduction in aplurality of passes of rough rolling. The variation in gain size isshown on the basis of the following meaning:

Large . . . maximum grain size/minimum grain size≧2

Medium . . . 2 maximum grain size/minimum grain size≧1.5

Small . . . maximum grain size/minimum grain size≧1.5

TABLE XI Number of times of Maximum rough rolling with rolling SheetAverage Sample rolling reduction of 20 reduction/ surface Edge crystalgrain Variation in Overall No. to 40% pass (%) state crack size (μm)grain size evaluation 8-1  0 10 A A 4.3 Large B 8-2  0 18 A A 4.2 LargeB 8-3  1 20 A A 4.2 Medium B 8-4  1 25 A A 4.2 Medium B 8-5  1 30 A A4.1 Medium B 8-6  1 40 A A 4.1 Medium B 8-7  1 44 B C 4.0 Medium C 8-8 2 20 A A 4.2 Small A 8-9  2 27 A A 4.1 Small A 8-10 2 30 A A 4.1 Small A8-11 2 36 A A 4.0 Small A 8-12 2 40 A A 4.0 Small A 8-13 2 43 B C 4.0Small C 8-14 3 20 A A 4.1 Small A 8-15 3 30 A A 4.0 Small A 8-16 3 40 AA 3.9 Small A 8-17 3 43 B C 3.9 Small A 8-18 4 20 A A 4.0 Small A 8-19 430 A A 4.0 Small A 8-20 4 35 A A 3.9 Small A 8-21 4 42 B C 3.9 Small C8-22 5 20 A A 4.0 Small A 8-23 5 30 A A 4.0 Small A 8-24 5 40 A A 3.8Small A 8-25 6 20 A A 4.0 Small A

Test Example 9-1

The same AZ91 blank (thickness, 4 mm) as that used in Test Example 7-1was prepared and then roughly rolled to a thickness of 1 mm underdifferent conditions to prepare a plurality of roughly rolled sheets.The roughly rolled sheets were finish-rolled to a final sheet thicknessof 0.5 mm under the same conditions to prepare magnesium alloy sheets.

The rough rolling was performed under the conditions in which thesurface temperature of the blank immediately before rough rolling was350° C., and the surface temperature of a rough reduction roll wascontrolled in the range of 200° C. to 230° C. During the rough rolling,the rolling reduction per pass was changed.

On the other hand, the finish rolling was performed under the conditionsin which the surface temperature of each roughly rolled sheetimmediately before finish rolling was 210° C. to 240° C., the surfacetemperature of a finish reduction roll was controlled in the range of150° C. to 180° C., and the rolling reduction per pass in the finishrolling was controlled to 15%.

Next, each of the finish-rolled sheets was heat-treated at 320° C. for30 minutes by the method as in Test Example 7-1 to form an evaluationsample. For these samples, the measurement of the average crystal grainsize, the evaluation of the sheet surface state, the evaluation of edgecracks, and the evaluation of variation in grain size were performed bythe same methods as in Test Example 6. Furthermore, the overallevaluation based on these evaluation results was carried out.

The number of times of rough rolling with a rolling reduction per passof 20% to 40% and the evaluation results are shown in Table XII. In thistable, the terms “Sheet surface state”, “Edge crack”, and “Variation ingrain size” mean the same as in Test Example 8.

TABLE XII Number of times of Maximum rough rolling with rolling SheetAverage Sample rolling reduction of 20 reduction/ surface Edge crystalgrain Variation in Overall No. to 40% pass (%) state crack size (μm)grain size evaluation 9-1  0 10 A A 5.0 Large B 9-2  0 18 A A 4.9 LargeB 9-3  1 20 A A 4.9 Medium B 9-4  1 25 A A 4.8 Medium B 9-5  1 30 A A4.7 Medium B 9-6  1 40 A A 4.5 Medium B 9-7  1 44 B C 4.5 Medium C 9-8 2 20 A A 4.9 Small A 9-9  2 27 A A 4.8 Small A 9-10 2 30 A A 4.7 Small A9-11 2 36 A A 4.6 Small A 9-12 2 40 A A 4.5 Small A 9-13 2 43 B C 4.5Small C 9-14 3 20 A A 4.9 Small A 9-15 3 30 A A 4.8 Small A 9-16 3 40 AA 4.6 Small A 9-17 3 43 B C 4.5 Small C 9-18 4 20 A A 4.9 Small A 9-19 430 A A 4.8 Small A 9-20 4 35 A A 4.6 Small A 9-21 4 42 B C 4.4 Small C9-22 5 20 A A 4.8 Small A 9-23 5 30 A A 4.7 Small A 9-24 5 40 A A 4.3Small A 9-25 6 20 A A 4.6 Small A

Test Example 9-2

A magnesium alloy blank having a different Al content from that in TestExample 9-1 was used for examining the influences of the temperature ofthe blank and the roll temperature in rough rolling by the same methodas in Test Example 9-1. The producing conditions other than the roughrolling conditions and the evaluation method for the magnesium alloysheets were the same as in Test Example 9-1. The Al content of themagnesium alloy blank was 9.8% by mass, and the Zn content thereof was1.0% by mass. The finish rolling conditions and the test results aresummarized in Table XIII.

TABLE XIII Number of times of Maximum rough rolling with rolling SheetAverage Sample rolling reduction of 20 reduction/ surface Edge crystalgrain Variation in Overall No. to 40% pass (%) state crack size (μm)grain size evaluation 9-26 0 10 A A 5.0 Large B 9-27 1 25 A A 4.9 MediumB 9-28 1 40 A A 4.6 Medium B 9-29 1 43 B C 4.6 Medium C 9-30 2 20 A A4.9 Small A 9-31 2 28 A A 4.8 Small A 9-32 2 38 A A 4.5 Small A 9-33 244 B C 4.4 Small C 9-34 3 20 A A 4.9 Small A 9-35 3 42 B C 4.5 Small C9-36 4 20 A A 4.9 Small A 9-37 4 43 B C 4.4 Small C 9-38 5 20 A A 4.9Small A 9-39 5 30 A A 4.7 Small A 9-40 5 38 A A 4.4 Small A

Summary of Test Examples 6 to 9

The results of Test Examples 6 to 9 reveal that rough rolling underappropriate conditions can produce a magnesium alloy sheet having smallvariation in grain size of the crystal grains, no problem such asdefects in the sheet surface and edge cracks, and excellent plasticworkability.

Test Example 10)

Magnesium alloy blanks (thickness, 4.0 mm) having a Mg-9.0% A1-1.0% Zn(% by mass) composition and a Mg-9.8% A1-1.0% Zn (% by mass) compositionwere prepared by twin-roll continuous casting. The centerlinesegregation produced in the magnesium alloy blanks had a maximum lengthof 50 μm in the thickness direction of the blanks. The magnesium alloyblanks were treated under the three types of conditions given below andthen rolled. Mg-9.0% A1-1.0% Zn composition (% by mass)

10-1 . . . Without solution treatment

10-2 . . . 405° C. for 1 hour (solution treatment)

10-3 . . . 405° C. for 10 hours (solution treatment)

Mg-9.8% A1-1.0% Zn composition (% by mass)

10-4 . . . Without solution treatment

10-5 . . . 405° C. for 1 hour (solution treatment)

10-6 . . . 405° C. for 10 hours (solution treatment)

Each of the magnesium alloy sheets prepared by the above-describedtreatments was rolled to a thickness of 0.6 mm under the followingconditions and then heat-treated under appropriate conditions to form asheet having an average crystal grain size of 5.0 μm.

<Rough Rolling: 4.0 mm to 1.0 mm>

Roll surface temperature: 200° C.

Sheet heating temperature: 330° C. to 360° C.

Rolling Reduction Per Pass: 20% to 25%<

Finish rolling: 1.0 mm to 0.6 mm>

Roll Surface Temperature: 180° C.

Sheet heating temperature: 230° C.

Rolling reduction per pass: 10% to 15%<

<Heat Treatment>

Annealing at 320° C. for 30 minutes

Next, a JIS 13B tensile test sample was prepared from each of the sheetsand subjected to a tensile test at a strain rate of 1.4×10⁻³ (s⁻¹) atroom temperature. Also, the alloy structure of a section of each sheetof 0.6 mm in thickness was observed to measure the amount (maximumlength in the thickness direction) of centerline segregation. The testmethods and meanings were as follows:

Tensile strength=load at breakage/(thickness of specimen×width of sheet)

Yield strength=measured at a proof strength of 0.2%

Yield ratio=yield strength/tensile strength

Elongation at breakage=(gage length when broken ends were placed backtogether−50 mm)/50 mm*1

*1: A so-called butt method for determining an elongation at breakagefrom a distance (50 mm) between the two gage marks previously set beforethe test and a distance between the two gage marks when the broken endsof a sample broken in the test were placed back together.

The results are shown in Table XIV.

TABLE XIV Centerline Tensile segregation strength Yield strengthElongation at Yield No. (μm) (MPa) (MPa) breakage (%) ratio (%) 10-1 30340 248 13 72.9 10-2 18 365 280 17 76.5 10-3 10 380 300 20 79.0 10-4 35348 255 12 73.2 10-5 19 370 284 16 76.8 10-6 12 386 305 20 79.0

It could be confirmed from Table XIV that solution treatment of themagnesium alloy blank prepared by the twin-roll continuous castingmethod decreases the length of centerline segregation in the thicknessdirection, thereby producing a magnesium alloy sheet having excellentmechanical properties. In particular, by using a magnesium alloy havinga high Al content, including a magnesium alloy corresponding to AZ91, amagnesium alloy sheet having more excellent mechanical properties can beproduced by solution treatment for a long time.

Test Example 11

Magnesium alloy blanks (thickness, 4.0 mm) having a Mg-9.0% A1-1.0% Zncomposition (% by mass) and a Mg-9.8% A1-1.0% Zn composition (% by mass)corresponding to AZ91 were prepared by twin-roll continuous casting.Each of these blanks was subjected to solution treatment at 405° C. for10 hours and then rolled to a thickness of 0.6 mm under the conditionsgiven below to prepare a magnesium alloy sheet. The centerlinesegregation produced in the resultant magnesium alloy sheets had amaximum length of 20 μm in the thickness thereof.

<Rough Rolling: 4.0 mm to 1.0 mm>

Roll surface temperature: 200° C.

Sheet heating temperature: 330° C. to 360° C.

Rolling reduction per pass: 20% to 25%<

Finish Rolling: 1.0 mm to 0.6 mm>

Roll surface temperature: 180° C.

Sheet heating temperature: 230° C.

Rolling reduction per pass: 10% to 15%

Next, each of the magnesium alloy sheets prepared by rolling under theabove-described conditions was treated under the three types ofconditions given below to form a sheet for evaluation.

<Heat Treatment>

(1) Without heat treatment after rolling

(2) Annealing at 230° C. for 1 minute

(3) Annealing at 320° C. for 30 minutes

Next, a JIS 13B tensile test sample was prepared from each of the sheetsand subjected to a tensile test at a strain rate of 1.4×10⁻³ (s⁻¹) atfour temperatures (room temperature, 150° C., 200° C., and 250° C.).Also, the alloy structure of a section of each sheet of 0.6 mm inthickness was observed before and after the tensile test. The testmethods and the meanings of terms were the same as in Test Example 10,and the description thereof is omitted.

The results are shown in Tables XV and XVI. Table XV shows the resultsof the test using the magnesium alloy sheets having the Mg-9.0% Al-1.0%Zn composition, and Table XVI shows the results of the test using themagnesium alloy sheets having the Mg-9.8% A1-1.0% Zn composition.

TABLE XV Heat Tensile Yield treatment strength strength Elongation atNo. after rolling Metal structure Test temperature (MPa) (MPa) breakage(%) 11-1 No Residual work strain  25° C. 420 360 1 to 3 11-2 No Residualwork strain 150° C. 190 140 30 to 90 11-3 No Residual work strain 200°C. 95 65  60 to 210 11-4 No Residual work strain 250° C. 52 33  65 to220 11-5 230° C. Partially recrystallized  25° C. 400 340 2 to 3 1 min11-6 230° C. Partially recrystallized 150° C. 200 158 40 to 60 1 min11-7 230° C. Partially recrystallized 200° C. 100 73  40 to 205 1 min11-8 230° C. Partially recrystallized 250° C. 60 40  80 to 190 1 min11-9 320° C. Completely recrystallized  25° C. 365 280 16 to 18 30 min 11-10 320° C. Completely recrystallized 150° C. 220 170 50 to 60 30 min 11-11 320° C. Completely recrystallized 200° C. 140 130 80 to 86 30 min 11-12 320° C. Completely recrystallized 250° C. 90 80 100 to 110 30 min

TABLE XVI Heat Tensile Yield treatment Test strength strength Elongationat No. after rolling Metal structure temperature (MPa) (MPa) breakage(%) 11-13 No Residual work strain  25° C. 428 368 1 to 2 11-14 NoResidual work strain 150° C. 195 145 34 to 88 11-15 No Residual workstrain 200° C. 100 70  65 to 200 11-16 No Residual work strain 250° C.56 35  67 to 210 11-17 230° C. Partially recrystallized  25° C. 410 3452 to 4 1 min 11-18 230° C. Partially recrystallized 150° C. 210 165 40to 65 1 min 11-19 230° C. Partially recrystallized 200° C. 108 77  50 to195 1 min 11-20 230° C. Partially recrystallized 250° C. 65 45  75 to203 1 min 11-21 320° C. Completely recrystallized  25° C. 368 285 16 to19 30 min 11-22 320° C. Completely recrystallized 150° C. 226 175 55 to65 30 min 11-23 320° C. Completely recrystallized 200° C. 145 129 84 to90 30 min 11-24 320° C. Completely recrystallized 250° C. 92 80 105 to114 30 min

<Structure of Magnesium Alloy Sheet Before Pressing>

Tables XV and XVI indicate that the sheets (11-9 to 11-12 or 11-21 to11-24) annealed at 320° C. for 30 minutes have no strain accumulated inthe magnesium alloy sheets by rolling work and are completelyrecrystallized. On the other hand, in the sheets (11-5 to 11-8 or 11-17to 11-20) annealed at 230° C. for 1 minute, the residual strain of thecrystal grains produced by rolling work partially remains. In addition,in the sheets (11-1 to 11-4 or 11-13 to 11-16) not heat-treated, theresidual strain of the crystal grains produced by rolling work remains.

<Structure of Magnesium Alloy Sheet after Plastic Deformation>

In the sheets completely recrystallized by annealing at 320° C. for 30minutes, the crystal grains in the structures of the sheets were notcoarsened by heating (250° C. or less) in tensile work, thereby causingsubstantially no change in the average crystal grain size before andafter the work. Therefore, it is supposed that in each of the sheets, aportion deformed by the tensile work is improved in hardness andstrength by the accumulated work strain, and a portion not deformed isnot changed in hardness and strength. On the other hand, in the sheets(not annealed or annealed at 230° C. for 1 minute) having the residualwork strain produced by rolling, the metal structures wererecrystallized by heating in tensile work to decrease strength andhardness. Furthermore, after the work, a portion not deformed isdecreased in strength, and a portion deformed is decreased or improvedin strength according to the degree of heating in the work. Therefore,if a magnesium alloy sheet contains a portion decreased in strength andhardness after working, it is impossible to stably produce a magnesiumalloy product having desired mechanical properties.

<High-Temperature Tensile Properties>

The sheets annealed at 320° C. for 30 minutes showed high tensilestrength, yield strength, and elongation at breakage at room temperatureand also showed high elongation at breakage at 200° C. and 250° C. Onthe other hand, the sheets having residual work strain showed abnormallyhigh elongation at breakage at 200° C. and 250° C. (superplasticphenomenon). However, there were very few sheets exhibiting such asuperplastic phenomenon, and the other sheets had low elongation atbreakage and caused damage such cracks and flaws during plastic working.Therefore, if there is large variation in elongation at breakage ofsheets, the products produced by plastic working of magnesium alloysheets have unstable quality.

These results reveal that a sheet having residual work strain is changedin metal structure by heating and deformation in plastic working at hightemperatures, and stable workability cannot be expected because thedegree of the change is unstable. On the other hand, a sheet having acompletely recrystallized metal structure is slightly changed in metalstructure after working, thereby stabilizing plastic workability andimproving the mechanical properties of a portion deformed by theworking. Furthermore, it is supposed that a portion not deformed alsomaintains the mechanical properties before working. Therefore, a sheetin which the work strain accumulated in rolling work has been relievedhas stable mechanical properties even in high deformation such as pressforming and is thus suitable for producing casing products by pressforming or the like.

Test Example 12

Next, casting, rough rolling, and finish rolling were carried out underthe conditions described in Test Example 11 to prepare magnesium alloysheets of 0.6 mm in thickness (Mg-9.0% A1-1.0% Zn and Mg-9.8% Al-1.0%Zn). After the finish rolling, each of the magnesium ally sheets wasannealed at 320° C. for 30 minutes to prepare an evaluation sample usedin a bending test. The bending test was a so-called three-point bendingtest in which each sample was supported at two points, and bendingpressure was applied to the sample by a forming tool (punch) from theside opposite the support points. The conditions of the bending test areshown below.

<Test Conditions>

Sample dimensions . . . width 20 mm, length 120 mm, thickness 0.6 mm

Test temperature . . . 25° C. (room temperature), 200° C., 250° C.

Tip angle of punch . . . 30°

Radius of punch (=bending radius of sample) . . . 0.5 mm, 1.0 mm, 2.0mm.

Support point distance . . . 30 mm

Penetration depth of punch . . . 40 mm

Penetration rate of punch . . . 1.0 m/min., 5.0 m/min

The test under the above-described conditions was performed to examinethe surface state and the amount of spring back of a bending-radiusportion of a sample. Also, the overall evaluation of a sample wasperformed on the basis of the surface state and the amount of springback. The term “spring back” means the phenomenon that the deformationof a sheet sample produced by a load applied from the punch remainsafter the load applied from the punch is removed. Namely, when theamount of spring back is large, deformability is decided as “poor”,while when the amount of spring back is small, deformability is decidedas “good”. Therefore, the ease of working of a sample can be decided byexamining the amount of spring back. The criteria for the surface stateand the amount of spring back are as follows:

<Criteria for Surface State>

No occurrence of cracks . . . A

Occurrence of fine cracks without breakage . . . B

Occurrence of breakage . . . C

<Criteria for Spring Back>

The spring back was evaluated by (angle formed by planes holdingbending-radius portion of sample with load applied from punch)−(angleformed by planes holding bending-radius portion without load applied) onthe basis of the following criteria:

Difference of 45° or more . . . large spring back

Difference of 10° to less than 45° . . . medium spring back

Difference of less than 10° . . . small spring back

<Overall Evaluation>

Surface state “C” . . . overall evaluation “C”

Surface state “A” and small spring back . . . overall evaluation “A”

Other cases . . . overall evaluation “B”

Furthermore, a bending characteristic value was defined as an indexindicating the degree of working. The bending characteristic value isrepresented by (bending radius (mm) of sample)/(thickness (mm) ofsample). As the bending radius of a sample decreases, local pressure isapplied to a bending-radius portion of a sample to easily produce damagesuch as cracks in the sample. As the thickness of a sample increases,the formability of the sample degrades to easily produce damage such ascracks. Therefore, a smaller bending characteristic value represented bythe above expression indicates high deformation under severe workingconditions.

The results of the evaluation of the surface state, spring back, andbending characteristic value, and the overall evaluation are shown inTables XVII and XVIII. Table XVII shows the results of the test usingthe magnesium alloy sheets having the Mg-9.0% A1-1.0% Zn composition,and Table XVIII shows the results of the test using the magnesium alloysheets having the Mg-9.8% A1-1.0% Zn composition.

TABLE XVII Bending Working Test radius rate Radius/ Spring Surface No.temperature (mm) (m/min) thickness back state Decision 12-1 25° C. 0.51.0 0.83 Large B B 12-2 25° C. 0.5 5.0 0.83 Large B B 12-3 25° C. 1.01.0 1.67 Large B B 12-4 25° C. 1.0 5.0 1.67 Large B B 12-5 25° C. 2.01.0 3.33 Large A B 12-6 25° C. 2.0 5.0 3.33 Large A B 12-7 200° C. 0.51.0 0.83 Small A A 12-8 200° C. 0.5 5.0 0.83 Small A A 12-9 200° C. 1.01.0 1.67 Small A A  12-10 200° C. 1.0 5.0 1.67 Small A A  12-11 200° C.2.0 1.0 3.33 Small A A  12-12 200° C. 2.0 5.0 3.33 Small A A  12-13 250°C. 0.5 1.0 0.83 Small A A  12-14 250° C. 0.5 5.0 0.83 Small A A  12-15250° C. 1.0 1.0 1.67 Small A A  12-16 250° C. 1.0 5.0 1.67 Small A A 12-17 250° C. 2.0 1.0 3.33 Small A A  12-18 250° C. 2.0 5.0 3.33 SmallA A

TABLE XVIII Bending Working Test radius rate Radius/ Spring Surface No.temperature (mm) (m/min) thickness back state Decision 12-19 25° C. 0.51.0 0.83 Large B B 12-20 25° C. 0.5 5.0 0.83 Large B B 12-21 25° C. 1.01.0 1.67 Large B B 12-22 25° C. 1.0 5.0 1.67 Large B B 12-23 25° C. 2.01.0 3.33 Large A B 12-24 25° C. 2.0 5.0 3.33 Large A B 12-25 200° C. 0.51.0 0.83 Small A A 12-26 200° C. 0.5 5.0 0.83 Small A A 12-27 200° C.1.0 1.0 1.67 Small A A 12-28 200° C. 1.0 5.0 1.67 Small A A 12-29 200°C. 2.0 1.0 3.33 Small A A 12-30 200° C. 2.0 5.0 3.33 Small A A 12-31250° C. 0.5 1.0 0.83 Small A A 12-32 250° C. 0.5 5.0 0.83 Small A A12-33 250° C. 1.0 1.0 1.67 Small A A 12-34 250° C. 1.0 5.0 1.67 Small AA 12-35 250° C. 2.0 1.0 3.33 Small A A 12-36 250° C. 2.0 5.0 3.33 SmallA A

Table XVII shows that in the samples of Mg-9.0% A1-1.0% Zn, the surfacestate was evaluated as “A” only in the bending test with a bendingradius of 2.0 mm, i.e., under mild working conditions (bendingcharacteristic value 3.33) (refer to Sample Nos. 12-15 and 12-16). Also,in the bending test at room temperature, spring back was large, andformability was low regardless of the bending radius and working rate(refer to Sample Nos. 12-1 to 12-6). On the other hand, in the bendingtest at 200° C. or more, spring back was small, and the surface statewas good regardless of the bending radius and the working rate (refer toSample Nos. 12-7 to 12-18).

On the other hand, as seen from Table XVIII, the samples of Mg-9.8%A1-1.0% Zn showed the completely same results as the samples of Mg-9.0%Al-1.0% Zn. Specifically, in the bending test at room temperature,formability was low (refer to Sample Nos. 12-19 to 12-24), while in thebending test at 200° C. or more, formability was high (refer to SampleNos. 12-25 to 12-36).

Test Example 13)

Casting, rough rolling, and finish rolling were carried out under theconditions described in Test Examples 11 and 12 to prepare magnesiumalloy sheets of 0.6 mm in thickness (Mg-9.0% A1-1.0% Zn and Mg-9.8%A1-1.0% Zn). Then, each of the magnesium ally sheets was treated underthe two types of conditions below to prepare evaluation samples used ina press test for examining the surface state of each sample afterpressing.

<Heat Treatment>

(1) No heat treatment after rolling

(2) Annealing at 320° C. for 30 minutes

<Conditions of Press Test>

Each sample was pressed by a servo pressing machine. Pressing wasperformed by pressing a parallelepiped upper mold against each samplewhich was placed on a parallelepiped lower mold to cover a recessedportion thereof. The upper mold is a parallelepiped of 60 mm by 90 mmand had the rounded four corners in contact with the sample, each of thecorners having a predetermined bending radius. Furthermore, a heater anda thermocouple were buried in each of the upper and lower molds so thatthe temperature condition of pressing could be controlled to a desiredtemperature.

<Test Conditions>

Bending radius of upper mold . . . 0.5 mm, 2.0 mm

Test temperature . . . 200° C., 250° C.

Working rate . . . 0.8 m/min, 1.7 m/min, 3.4 m/min, 5.0 m/min

After pressing under the above-described conditions, the surface stateof a bending-radius portion of each sample was examined. The results areshown in Tables XIX and XX. Table XIX shows the results of the testusing the magnesium alloy sheets having the Mg-9.0% A1-1.0% Zncomposition, and Table XX shows the results of the test using themagnesium alloy sheets having the Mg-9.8% A1-1.0% Zn composition. Thesurface state means the same as in Test Example 12, and the bendingcharacteristic value was determined by (bending radius of uppermold)/(thickness of sample).

TABLE XIX Working Bending Heat treatment Test Bending rate radius/Surface No. after rolling temperature radius (mm) (m/min) thicknessstate 13-1  No 200° C. 0.5 0.8 0.83 C 13-2  No 200° C. 2.0 0.8 3.33 B13-3  No 200° C. 0.5 1.7 0.83 C 13-4  No 200° C. 2.0 1.7 3.33 B 13-5  No200° C. 0.5 3.4 0.83 C 13-6  No 200° C. 2.0 3.4 3.33 B 13-7  No 200° C.0.5 5.0 0.83 C 13-8  No 200° C. 2.0 5.0 3.33 C 13-9  320° C., 30 min200° C. 0.5 0.8 0.83 A 13-10 320° C., 30 min 200° C. 2.0 0.8 3.33 A13-11 320° C., 30 min 200° C. 0.5 1.7 0.83 B 13-12 320° C., 30 min 200°C. 2.0 1.7 3.33 A 13-13 320° C., 30 min 200° C. 0.5 3.4 0.83 B 13-14320° C., 30 min 200° C. 2.0 3.4 3.33 A 13-15 320° C., 30 min 200° C. 0.55.0 0.83 C 13-16 320° C., 30 min 200° C. 2.0 5.0 3.33 A 13-17 No 250° C.0.5 0.8 0.83 B 13-18 No 250° C. 2.0 0.8 3.33 A 13-19 No 250° C. 0.5 1.70.83 B 13-20 No 250° C. 2.0 1.7 3.33 A 13-21 No 250° C. 0.5 3.4 0.83 C13-22 No 250° C. 2.0 3.4 3.33 A 13-23 No 250° C. 0.5 5.0 0.83 C 13-24 No250° C. 2.0 5.0 3.33 B 13-25 320° C., 30 min 250° C. 0.5 1.7 0.83 A13-26 320° C., 30 min 250° C. 2.0 1.7 3.33 A 13-27 320° C., 30 min 250°C. 0.5 3.4 0.83 A 13-28 320° C., 30 min 250° C. 2.0 3.4 3.33 A 13-29320° C., 30 min 250° C. 0.5 5.0 0.83 A 13-30 320° C., 30 min 250° C. 2.05.0 3.33 A

TABLE XX Working Bending Heat treatment Test Bending rate radius/Surface No. after rolling temperature radius (mm) (m/min) thicknessstate 13-31 No 200° C. 0.5 0.8 0.83 C 13-32 No 200° C. 2.0 0.8 3.33 B13-33 No 200° C. 0.5 1.7 0.83 C 13-34 No 200° C. 2.0 1.7 3.33 B 13-35 No200° C. 0.5 3.4 0.83 C 13-36 No 200° C. 2.0 3.4 3.33 B 13-37 No 200° C.0.5 5.0 0.83 C 13-38 No 200° C. 2.0 5.0 3.33 C 13-39 320° C., 30 min200° C. 0.5 0.8 0.83 A 13-40 320° C., 30 min 200° C. 2.0 0.8 3.33 A13-41 320° C., 30 min 200° C. 0.5 1.7 0.83 B 13-42 320° C., 30 min 200°C. 2.0 1.7 3.33 A 13-43 320° C., 30 min 200° C. 0.5 3.4 0.83 B 13-44320° C., 30 min 200° C. 2.0 3.4 3.33 A 13-45 320° C., 30 min 200° C. 0.55.0 0.83 C 13-46 320° C., 30 min 200° C. 2.0 5.0 3.33 A 13-47 No 250° C.0.5 0.8 0.83 B 13-48 No 250° C. 2.0 0.8 3.33 A 13-49 No 250° C. 0.5 1.70.83 B 13-50 No 250° C. 2.0 1.7 3.33 A 13-51 No 250° C. 0.5 3.4 0.83 C13-52 No 250° C. 2.0 3.4 3.33 A 13-53 No 250° C. 0.5 5.0 0.83 C 13-54 No250° C. 2.0 5.0 3.33 B 13-55 320° C., 30 min 250° C. 0.5 1.7 0.83 A13-56 320° C., 30 min 250° C. 2.0 1.7 3.33 A 13-57 320° C., 30 min 250°C. 0.5 3.4 0.83 A 13-58 320° C., 30 min 250° C. 2.0 3.4 3.33 A 13-59320° C., 30 min 250° C. 0.5 5.0 0.83 A 13-60 320° C., 30 min 250° C. 2.05.0 3.33 A

Table XIX indicates that among the samples having the Mg-9.0% Al-1.0% Zncomposition, the samples not heat-treated after finish rolling producedcracks or flaws in the surfaces during pressing at a sample temperatureof 200° C. In particular, cracks were produced in the surfaces in highdeformation with a bending characteristic value of 0.83. The samesamples also produced cracks or flaws in the surfaces in the press testat 250° C. with high deformation (bending characteristic value of 0.83).On the other hand, the samples annealed at 320° C. for 30 minutes afterfinish rolling showed a good surface state in pressing at a sampletemperature of 200° C. and a high working rate (refer to Sample Nos.13-9 and 13-10) and in pressing with a bending characteristic value of3.33 (refer to Sample Nos. 13-10, 13-12, 13-14, and 13-16). Theseannealed samples also showed a good surface state in pressing at 250° C.regardless of the bending characteristic value and the working rate.

Table XX indicates that the samples of Mg-9.8% A1-1.0% Zn showedsubstantially the same test results as the samples of Mg-9.0% A1-1.0%Zn. Namely, the samples annealed at 320° C. for 30 minutes showed a goodsurface state after pressing as compared with the samples not annealed.Furthermore, the higher the pressing temperature, the better the surfacestates of the samples. In particular, it was found that in pressing anannealed magnesium alloy sheet at 250° C., press formability is higheven in high deformation (characteristic bending value of 0.83) at aworking rate of 5.0 m/min.

Summary of Test Examples 11 to 13

The results of Test Examples 11 to 13 reveal that when the structure ofa magnesium alloy sheet is recrystallized by heat treatment at a propertemperature after rolling, formality is stabilized. The cause ofstabilizing formability is supposed to be that the metal structure isnot much changed by heating in plastic working (including pressing)because the metal structure is recrystallized before plastic working.

INDUSTRIAL APPLICABILITY

The method for producing the magnesium alloy sheet of the presentinvention can be suitably used for producing a magnesium alloy sheethaving excellent plastic workability, particularly press workability. Inaddition, the magnesium alloy sheet of the present invention can besuitably used as an alloy material required to have a light weight andhigh mechanical properties.

1-10. (canceled)
 11. A magnesium alloy sheet produced by a methodcomprising a step of: rolling a magnesium alloy blank with a reductionroll; wherein the rolling includes controlled rolling in which thesurface temperature Tb (° C.) of the blank immediately before insertioninto the reduction roll satisfies the following expression:33M+135≦Tb≦8.33×M+165 wherein 1.0≦M≦10.0 M and M (% by mass) is the Alcontent in a magnesium alloy constituting the blank; and the surfacetemperature Tr of the reduction roll is 150° C. to 180° C.
 12. Themagnesium alloy sheet according to claim 11, wherein an amount ofsegregation present at a centerline in a thickness direction of themagnesium alloy sheet is 20 μM in the thickness direction.
 13. Themagnesium alloy sheet according to claim 11 or 12, wherein the magnesiumalloy has an Al content M of 8.5 to 10.0% by mass and further contains0.5 to 1.5% by mass of zinc, and the magnesium alloy sheet has a tensilestrength of 360 MPa or more, a yield strength of 270 MPa or more, and anelongation at breakage of 15% or more at room temperature.
 14. Themagnesium alloy sheet according to claim 11, wherein a yield ratio is75% or more.
 15. The magnesium alloy sheet according to claim 11 or 12,wherein the magnesium alloy has an Al content M of 8.5 to 10.0% by massand further contains 0.5 to 1.5% by mass of zinc, and the magnesiumalloy sheet has a tensile strength of 120 MPa or more and an elongationat breakage of 80% or more at 200° C., and a tensile strength of 90 MPaor more and an elongation at breakage of 100% or more at 250° C.
 16. Themagnesium alloy sheet according to claim 11 or 12, wherein the magnesiumalloy has an Al content M of 8.5 to 10.0% by mass and further contains0.5 to 1.5% by mass of zinc, and the magnesium alloy sheet producesneither crack nor flaw in a surface in a bending work under theconditions of 200° C. or more and a bending characteristic value(bending radius R/thickness t) of 1.0 or less.
 17. The magnesium alloysheet according to claim 11 or 12, wherein the magnesium alloy has an Alcontent M of 8.5 to 10.0% by mass and further contains 0.5 to 1.5% bymass of zinc, and the magnesium alloy sheet produces neither crack norflaw in a surface in a press work under the conditions of 200° C. ormore and a bending characteristic value (bending radius R/thickness t)of 1.0 or less.